Fallen Chinese politician Bo Xilai has launched a scathing attack on a key witness in his corruption trial, saying even the most stupid official knows not to discuss bribery where they can be overheard.

The scandal which has rocked the ruling Communist Party was triggered by the death of a British businessman -- for which Bo's wife Gu Kailai was convicted of murder -- and the courtroom drama has gripped the nation.

Bo's performance has been defiant, denying bribe-taking and embezzlement, with accusations of abuse of power still to be addressed as the trial -- widely expected to be only two days long -- moved into an unexpected third day.

Wang Zhenggang, a former planning official in Dalian, where Bo was the mayor in the 1990s, told the court the politician had telephoned Gu in front of him and told her to take delivery of five million yuan ($820,000) from a local government construction project.

Bo said the claim of a phone call did not make sense. "It is not even what the most stupid corruption offender would do," he said.

"Corrupt offenders with even the lowest IQ would ask who else in Dalian was aware of the money."

He added: "All those who know me know that I ask them to switch off their mobile phones before I speak. I am quite cautious."

The comments appeared in regular but delayed transcripts of the hearings the Intermediate People's Court in Jinan, in eastern China, is providing on its verified account on Sina Weibo, China's equivalent of Twitter.

But no live audio or video is available, and nor are any foreign media or independent observers present in court.

The scandal erupted in advance of a generational shift of power atop the factionalised Communist Party, and Bo's feisty performance during his trial has astonished a public unfamiliar with the open airing of top-level intrigue.

Revelations of private jet flights, luxury villas and rare animal meats have held Chinese readers spellbound, while Bo has showed open disdain for prosecution witnesses, including Gu, whom he derided as "insane".

Nonetheless analysts still believe a guilty verdict and long prison sentence have been agreed beforehand.

State-run Chinese media heaped praise on the trial, calling it a landmark in the history of Chinese jurisprudence.

"This degree of transparency has not happened before," the Global Times newspaper, which has close ties to the Communist Party, said in an editorial.

"This will create a precedent that will bring lasting impact to the future trials of sensitive cases."

It described the Weibo postings as a "live feed" and said it "served as an important guarantee of a fair trial for Bo in accordance with the law".

"The live show has addressed various doubts and rumours in and outside China, the paper said. "It demonstrated that the authorities are ready to receive more public scrutiny."

Bo was once head of the southwestern megacity of Chongqing, and tipped to join the highest echelons of Chinese political power.

But while the trial contents have been seen as astonishing, nothing has been said publicly of Bo's links with other top Communist leaders.

The hearings are a stark contrast to previous Chinese political trials in which most defendants have humbly confessed their crimes amid opaque court proceedings.

Calls by ordinary Chinese citizens for greater official transparency have been mounting, the Global Times said.

"For a while, the Chinese public has been complaining about injustice in constant news reports of scandals or social issues," the paper said.

"The open and transparent trial of Bo provided a different picture to the public, which will significantly change the image of the judicial system.

"The most important thing now is to have a fair trial for Bo's case, which will naturally boost the public's confidence," the paper said.